4-3sin^2x-4cosx=3cos^2x-4cosx+1 co sxE[cos2pai/3,1]cosx=4/(2x3)=2/3 ;2/3E[-1/2,1]最小值(4x1X3-16)/(4x3)=-1/3cosX=-1/2时y=3.75 Cosx=l时y=0 最大值3.75yE[-1/3,3.75]